1. Field of the Invention
The present invention is generally drawn to the chemical cleaning of iron and corrosion buildup in a steam generator and more particularly to a low temperature chemical cleaning of such unwanted buildup.
2. Description of the Prior Art
Previous low temperature chemical cleaning applications performed on NSSS steam generators (SG) were based on the research and development of the process cleaning steps conducted by the Electric Power Research Institute (EPRI) and Steam Generator Owners Group (SGOG) in the April 1983 (EPRI Report NP-3009) entitled “Steam Generator Chemical Cleaning Process Development” which is hereby included in this application by reference thereto and with a copy of the report enclosed. The application technique employed for the EPRI/SGOG chemical cleaning required the utility to be shutdown and the SG drained prior to initiation of the cleaning application so that connections could be made to inject the cleaning solutions. Since the solutions required heating and/or cooling based on the sequence of steps to be applied, different methods of achieving process temperature control have been implemented always using an external heat exchanger source requiring a considerable amount of equipment and manpower to perform the cleaning application successfully. A schematic of such a prior art chemical cleaning setup is seen in FIG. 1.
The above cited EPRI report teaches that low temperature chemical cleaning be conducted at 200 degrees ° F. and is stated to be effective for iron removal from a steam generator and for passivation of the cleaning surfaces. It recommends different temperatures, chemical formulations and exposure times for copper removal, cleaning of dents, crevices etc. The report was complied using a mock steam generator and did not test the use of nitrogen sparging. It did disclose various formulations of chemicals for effective low temperature cleaning but only after full shut-down of the steam generator.
Certain patents also teach the use of chemical cleaning. Some of these patents use chemical cleaning in conjunction with pressure pulses in the solution to improve cleaning (U.S. Pat. No. 5,841,826) passivation of cleaning surfaces (U.S. Pat. No. 5,587,025) and a specific formulation for chemical cleaning of metal compounds containing iron (U.S. Pat. No. 5,575,863).
However, the cited references fail to teach the use of chemical cleaning during a Mode 5 (200 degrees F.) steam generator shut-down operation with nitrogen sparging to mix the chemicals and obviate the need for numerous recycles and thus requiring no external heat source.